Various nonwoven fabrics have been proposed as materials of air filters for removing dust and liquid filters. Especially in recent years, thermo-compression type long fiber nonwoven fabrics excellent in stiffness are suitably used as pleated filters. Pleated filters allow low filtration air velocities since they can have wide filtration areas, and have such advantages as higher dust collection capability and lower mechanical pressure loss.
However, the conventional thermo-compression type long fiber nonwoven fabrics do not have sufficient collection capability, since the fiber diameters of the fibers constituting the nonwoven fabrics are about 10 μm at the smallest.
For example, patent reference 1 proposes a composite long fiber nonwoven fabric for filters consisting of fibers with a special cross sectional form. According to the technique, the nonwoven fabric for filters can have better mechanical properties and higher dimensional stability, but it cannot sufficiently collect dust particles with particle sizes smaller than several micrometers, since the fiber diameter of the component fibers is 2 to 15 decitex (dtex), that is, about 13 μm at the smallest.
Further, patent reference 2 proposes a nonwoven fabric for filters, in which multiplex nonwoven fabrics are laminated. The technique allows easy production of a nonwoven fabric for filters having a high unit area weight and also allows the production of a nonwoven fabric for filters with excellent air permeability. However, since the proposed nonwoven fabric is obtained by laminating and integrating a nonwoven fabric with a fiber diameter of 7 to 20 μm and a nonwoven fabric with a fiber diameter of 20 to 50 μm, etc., the obtained nonwoven fabric cannot sufficiently collect dust particles with particle sizes smaller than several micrometers.
On the other hand, as nonwoven fabrics for filters having higher dust collection performance, various nonwoven fabrics for filters containing ultra-fine fibers are proposed.
For example, patent reference 3 proposes a nonwoven fabric for filters, in which a nonwoven fabric with a low melting point and a nonwoven fabric containing ultra-fine fibers are laminated, wherein the nonwoven fabric with a low melting point is molten for integration. This technique allows the ultra-fine fibers to be contained in a nonwoven fabric, without being molten, and because of this feature, since fine voids can be kept among the fibers in the nonwoven fabric, a nonwoven fabric with excellent dust collection performance can be produced. However, this technique has such problems that since the ultra-fine fibers do not contribute to the integration of the nonwoven fabric at all, the ultra-fine fibers are liable to come off from the nonwoven fabric and that the rate of ultra-fine fibers cannot be made high. Furthermore, this technique is not excellent in view of productivity either, since the ultra-fine fibers are obtained by treating a nonwoven fabric likely to be split into ultra-fine fibers by a high pressure liquid flow, needle punching or buckling treatment.
Moreover, patent reference 4 proposes a nonwoven fabric for filters with a unit area weight of 10 to 50 g/m2, consisting of a nonwoven fabric composed of ultra-fine fibers with a fiber diameter of 1 to 6 μm and a nonwoven fabric composed of long fibers with a fiber diameter of 10 to 30 μm. This technique can provide a nonwoven fabric small in the leak of powder when coffee powder or the like is extracted. However, since the nonwoven fabric provided by this technique has a unit area weight of about 10 to about 50 g/m2, it does not have the sufficient strength for allowing the use as an industrial filter. Further, this technique requires that the apertures on the surface of the long fiber nonwoven fabric are covered with the ultra-fine fibers. So the production method is complicated. Furthermore, the nonwoven fabric of this technique consists of the ultra-fine fiber nonwoven fabric as a melt blown nonwoven fabric and the long fiber nonwoven fabric as a spunbonded nonwoven fabric and the raw material is a polyester resin or a polyolefin resin. However, the use of a polyester resin as the raw material has such a problem that since the polyester of the melt blown nonwoven fabric is often insufficient in oriented crystallization, the sheet is cured or remarkably shrunken in the case of thermal bonding. Further, the use of the nonwoven fabric as a filter has such a problem that in a high temperature service environment, the sheet is cured or shrunken. On the other hand, the use of a polyolefin resin as the raw material has such problems that heat resistance is low since the melting point is low, and that since the sheet is soft in touch the use of it especially as a pleated filter is not preferred. Moreover, in the case where the resin of the melt blown nonwoven fabric is different from that of the spunbonded nonwoven fabric there is a problem that since the compatibility between the resins is insufficient the integration by thermal bonding is difficult.    [Patent reference 1] JP2001-276529A    [Patent reference 2] JP2004-124317A    [Patent reference 3] JP2001-248056A    [Patent reference 4] JP2004-154760A